The power-ful problem

A body is initially at rest. It undergoes one-dimensional motion with constant power. If its displacement, x is related to time, t as x = kt^p, where k is a constant, then what is the value of p?


The answer is 1.5.

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2 solutions

Vidhan Singh
Jun 2, 2015

P=ma.v

Given x=kt^p

So, a=ckt^(p-2) c=p(p-1) v=pkt^(p-1)

 ma.v=p=constant (power is contant)

Hence       t^(p-1+p-2) = 1

2p-3=0 , p= 1.5

Prakhar Gupta
Jun 2, 2015

From the definition of Power in Mechanics, we can write the relation that:- P = F v P = Fv P = m a v P = mav Using the relation that a = v d v d x a = v\dfrac{dv}{dx} . P = m v d v d x v P = mv\dfrac{dv}{dx}v P = m v 2 d v d x P = mv^{2}\dfrac{dv}{dx} Integrating the above expression we get:- 0 x P d x = 0 v m v 2 d v \int_{0}^{x} Pdx = \int_{0}^{v} mv^{2}dv P x = m v 3 3 Px = \dfrac{mv^{3}}{3} 3 P x m = v 3 \dfrac{3Px}{m} = v^{3} d x d t = ( 3 P x m ) 1 3 \dfrac{dx}{dt} = \Big( \dfrac{3Px}{m}\Big) ^{\frac{1}{3}} d x x 1 3 = ( 3 P m ) 1 3 d t \dfrac{dx}{x^{\frac{1}{3}}} = \Big( \dfrac{3P}{m} \Big)^{\frac{1}{3}} dt Again integrating both sides we get:- 0 x d x x 1 3 = 0 t ( 3 P m ) 1 3 d t \int_{0}^{x}\dfrac{dx}{x^{\frac{1}{3}}} = \int_{0}^{t} \Big( \dfrac{3P}{m}\Big)^{\frac{1}{3}} dt 3 x 2 3 2 = ( 3 P m ) 1 3 t \dfrac{3x^{\frac{2}{3}}}{2} = \Big( \dfrac{3P}{m}\Big)^{\frac{1}{3}} t Now cubing both sides:- 27 x 2 8 = 3 P m t 3 m \dfrac{27x^{2}}{8} = \dfrac{3Pmt^{3}}{m} x 2 = 8 P t 3 9 m x^{2} = \dfrac{8Pt^{3}}{9m} x = 2 2 3 P m t t x = \dfrac{2\sqrt{2}}{3} \sqrt{\dfrac{P}{m}}t\sqrt{t} Hence we can write that:- x = k t 1.5 x = kt^{1.5}

There is a typo in the fourth line of your solution. It should read as Pdx=mv^2dv. Also there is an extra m in the numerator in the last but 4 line.

Somsubhra Ghosh - 6 years ago

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Thanks, it is corrected now.

Prakhar Gupta - 6 years ago

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